1. Field of the Invention
This invention relates to the production of anhydrous aluminium chloride and more particularly to an improved process for the production of this material from aluminium chloride hexahydrate by dehydration followed by chlorination.
2. Description of the Prior Art
Currently aluminium is produced from bauxite using a combination of the Bayer and Hall-Heroult processes. The Bayer process produces high purity alumina from bauxite which, in the Hall-Heroult process, is continuously dissolved in a fluoride bath (principally cryolite) and reduced electrolytically, and molten aluminium is collected above the carbon cathode which forms the base of the furnace. The electrolytic reduction of alumina which is the basis for the Hall-Heroult process uses very large amounts of electrical energy, typically 13-18 MWh per tonne of metal and also consumes carbon anodes through oxidation by oxygen evolved during the electrolysis.
In recent years, production of aluminium by the electrolysis of anhydrous aluminium chloride dissolved in a molten electrolyte composed of one or more halides of alkali and/or alkaline earth metals has received considerable attention (U.S. Pat. No. 3,725,222) as this method offers certain potential advantages over the Hall-Heroult process. The advantages which include operation at a lower temperature, lower power consumption (9.5 MWh per tonne of metal) and non-consumption of carbon electrodes indicate significant cost savings over the conventional Hall-Heroult process.
A pre-requisite to the chloride electrolysis process is the production of high purity anhydrous aluminium chloride feed for the electrolysis stage. Although the potential of aluminium chloride electrolysis as a method of producing aluminium metal has been recognized for a while, the development of the process to a commercial scale has been inhibited due to the unavailability of an economic process for the production of high purity anhydrous aluminium chloride. Many processes have been proposed for the production of anhydrous aluminium chloride from bauxite and alumina but to date the suggested methods have not satisfied the objective of commercial production of high purity anhydrous aluminium chloride. Chlorination of bauxite results in aluminium chloride contaminated with the chlorides of iron, titanium and silicon and the separation of these impurities from aluminium chloride requires complex and costly processing steps. On the other hand, the chlorination of Bayer, alumina, although it results in high purity aluminium chloride, requires an expensive starting material and unlike direct electrolysis of alumina in the Hall cell introduces an extra step of chlorination. Furthermore, Bayler alumina contains about 0.5% of sodium oxide, which during chlorination, will consume chlorine to produce NaAlCl.sub.4. The formation of the latter compound would not only create problems during chlorination but also result in some loss of chlorine values. Chlorination is generally carried out above 600.degree. C. (West German Pat. No. 1,229,056) using high purity, as such expensive, carbon monoxide as reductant. An alternative to carbon monoxide as reductant is the chlorination of porous alumina particles being intermixed, and preferably coated or impregnated, with carbon to provide an average carbon content of about 15-24% by weight and having an average hydrogen content of less than 0.5% by weight and desirably of less than 0.3% and preferably less than 0.1% (U.S. Pat. No. 3,842,163). The preferred chlorination temperatures are in the range 650.degree.-700.degree. C.
Unlike many hydrated compounds, aluminium chloride hexahydrate when heated to remove combined water does not yield anhydrous aluminium chloride. Instead, Al.sub.2 O.sub.3, HCl and H.sub.2 O are produced according to the following reaction: EQU 2AlCl.sub.3.6H.sub.2 O.fwdarw.Al.sub.2 O.sub.3 +6HCl+9H.sub.2 O
According to the studies which have been carried out on the dehydration reaction, decomposition takes place at 180.degree. C. and heating in a stream of HCl does not prevent the above hydrolysis reaction. Very little loss in weight is observed below this temperature, particularly at 100.degree. C.
The literature contains very little information on the dehydration of AlCl.sub.3.6H.sub.2 O to AlCl.sub.3. According to Heap and Newbery (British Pat. Nos. 130,626 and 131,039) dehydration of aluminium chloride hexahydrate can be carried out by reacting it with phosgene which has a great affinity for water. These patents do not give any experimental details and results except indicating that aluminium chloride hexahydrate was heated at 100.degree. C. before reacting it with phosgene and that hydrogen was excluded from the chlorinating gas mixture in order to avoid formation of water.
Thus it may be seen that the dehydration of hexahydrate according to the teaching of the above patents would result in the production of a large excess of HCl. The amount of HCl produced would be about four times that required to produce aluminium chloride hexahydrate from bauxite or other aluminum containing materials as shown below, representing an overall loss of chlorine and CO from the system. EQU 2AlCl.sub.3.6H.sub.2 O+12CO+12Cl.sub.2 .fwdarw.2AlCl.sub.3 +12CO.sub.2 +24HCl EQU Al.sub.2 O.sub.3 +6HCl+9H.sub.2 O.fwdarw.2AlCl.sub.3. 6H.sub.2 O
Although one of the patents (British Pat. No. 131,039) discloses the use of producer gas (practically free from hydrogen), production of hydrogen-free producer gas is relatively expensive and the presence of nitrogen in the producer gas, although it may not affect the dehydration reaction, would have an adverse effect on the condensation of AlCl.sub.3 from gas mixture containing AlCl.sub.3, CO, CO.sub.2 and N.sub.2 due to the dilution effect of nitrogen further lowering the partial pressure of AlCl.sub.3.
East German Pat. No. 130,656 describes a two-stage process for the production of anhydrous aluminium chloride wherein, in the first stage aluminium chloride hexahydrate is heated at a temperature between 100.degree. and 500.degree. C. to remove a major proportion of water and HCl. In the second stage the first stage product (a basic aluminium chloride) is heated at a temperature between 600.degree. and 900.degree. C. to produce anhydrous aluminium chloride which is removed by a carrier gas, which preferably contains gaseous HCl, and leave a residue of .alpha.-Al.sub.2 O.sub.3. Only some 10-20% of the aluminium present in the starting material emerges as the anhydrous chloride product.